(1) Field of the Invention
The present invention relates to a semiconductor laser apparatus for use in an optical pickup apparatus or the like, and to a production method thereof.
(2) Description of the Related Art
Optical pickup apparatuses include, for example: a semiconductor laser element for emitting a laser beam onto a target optical disc; a photodetector element for receiving the laser beam reflected off the optical disc, and a signal processing circuit for processing an electric signal generated by the photodetector element based on the received laser beam.
In recent years, for the purpose of downsizing the apparatus and simplifying the alignment adjustment for each optical element, what is called an optical-system-integrated-driving-type semiconductor laser apparatus in which the above-mentioned semiconductor laser element, photodetector element, and signal processing circuit are deposited on a piece of semiconductor substrate has been developed (Japanese Laid-Open Patent Application No. 64-27288).
When, as is the case with the optical-system-integrated-driving-type semiconductor laser apparatus, a semiconductor laser element and a photodetector element are formed on a same substrate, it is required to fix the semiconductor laser element at a given position with great precision. This is because the accuracy in the position of the semiconductor laser element relative to the optical disc to which the laser beam is emitted greatly affects the performance of the apparatus. That is to say, the apparatus's performance is degraded if the semiconductor laser element is positioned and attached with low accuracy.
Japanese Laid-Open Patent Application No. 9-326535 discloses a technology intended to secure the accuracy in attaching the semiconductor laser element to the substrate. According to the technology, a recess is formed in the substrate in advance, and the bump electrode of the semiconductor laser element, which is a source of outgoing laser beams, is inserted in the recess.
Although the technology disclosed in Japanese Laid-Open Patent Application No. 9-326535 has succeeded in increasing the accuracy in attachment of the semiconductor laser element to some extent, the positioning and attachment of the semiconductor laser element along a substrate-thickness direction (a recess-depth direction) is not satisfactory yet for the following reasons. The semiconductor laser element is soldered onto electrodes composed of gold (Au) that have been formed in advance on a surface of the semiconductor substrate. During the bonding process by soldering, a chemical reaction occurs between tin (Sn) element in the solder and Au element in the electrodes, and a distance between the bottom face and the semiconductor laser element is deviated from a desired distance.
In the semiconductor laser apparatus of an optical pickup apparatus, a laser beam emitted from the semiconductor laser element is reflected off a reflection unit placed near the semiconductor laser element, and is output upward outside the apparatus in the substrate-thickness direction. Here, if the semiconductor laser element has been deviated from a desired position along the substrate-thickness direction, the position on the reflection unit at which the emitted laser beam reaches is also deviated from a desired position. This results in a deviation of the reflected laser beam along the direction of the main surface of the apparatus. When such deviations are expected in the manufacturing process of the semiconductor laser apparatus, it is necessary to conduct a positional adjustment of an optical apparatus (for example, a hologram) that is deposited between the semiconductor laser element and the optical disc, for each semiconductor laser apparatus. This positional adjustment, however, is cumbersome and complicated.
As described above, in the semiconductor laser apparatus, a laser beam emitted from the semiconductor laser element is reflected off a reflection unit placed near the semiconductor laser element, and is output upward outside the apparatus in the substrate-thickness direction. It is supposed in both the above-mentioned documents (Japanese Laid-Open Patent Applications No. 64-27288 and No. 9-326535) that an Au layer is used as the reflection layer for reflecting the laser beams, assuming that the laser beams emitted from the semiconductor laser element have a wavelength of red to infrared light. The Au layer, however, is not suited for the blue light, which can also be emitted from the semiconductor laser element. This is because the reflectance of the Au layer for the blue light is lower than that for the red to infrared light due to the material characteristics. That is to say, on the assumption that the semiconductor laser apparatus may emit blue light, it is indispensable to adopt a reflection layer for reflecting blue light with high reflectance.
It is known conventionally that a reflection layer composed of Al has high reflectance in reflecting blue light. However, possibilities of using an Al layer as a reflection layer in semiconductor laser apparatuses have not been fully discussed, nor has been proposed the process for achieving it.